Pressure and strain measurement during hot extrusion of aluminium
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The aluminium extrusion process is based on simple principles, but allows profiles with the most complex cross-sectional shapes to be produced at a very high rate. One of the greatest challenges of aluminium extrusion is to control material flow and dimensional variability of thin-walled high-strength profiles, for which the demand is growing. The die outlet geometry and the temperatures of the billet and tools must be carefully tuned in order to secure satisfactory material flow conditions. Due to the high pressures in the container, the deformation of the extrusion dies and the distortion of the die outlet may be significant and must be compensated for. The thermal conditions in the extrusion press must also be controlled. Even in the age of numerical modelling much trial and error is needed to make certain that the product satisfies the customer requirements. If simple and effective process control could be implemented, the cost and dimensional variability of extruded profiles could be significantly reduced. This would not only secure the continued use of extruded profiles in old markets but also open new ones. The main objective of this study has been to establish useful and simple methods for measuring the pressure at the interface between the die and the billet and the deformation of the die during hot extrusion of aluminium. Pressure measurement data may be used to establish a better understanding of the extrusion process and to carefully evaluate the many numerical extrusion models that are presently being developed and refined for the purpose of predicting profile shape and properties. A most important task is the evaluation of constitutive models used to describe bulk material and friction behaviour. The requirements for such models should be viewed in relation to common flow instability phenomena such as buckling. Sensors may in the future be integrated in intelligent extrusion dies in order to make certain that temperature and flow condition changes are as small as possible and to prevent overloading of extrusion dies during production. It is of the utmost importance that dimensional variability is detected early. This study has consisted of many parts. The first step was a careful evaluation of sensor designs using the Capacitec HPC-75 high-temperature capacitive displacement probes and the Capacitec 4004 amplifier series. The feasibility of high- and low-temperature pressure measurement was demonstrated through various types of compression testing. The capacitive sensors were repeatedly tested in a hot air furnace to 650 ºC, and results were satisfactory for all but one of the sensors. The sensor sensitivity to temperature changes that occur during extrusion is usually less than 10 % of the full sensor response. The pressure sensors have been repeatedly tested in several dies for aluminium rod extrusion. The feasibility of, and a method for, performing useful measurements in the high-temperature extrusion environment have been demonstrated. The measurement accuracy is better than ± 10 % of full scale of 200 MPa when the effects of temperature changes are compensated for. The measurement repeatability is of a similar magnitude for genuinely replicated measurements. The measurement resolution is better than 1 %. It is firmly believed that the measurement and calibration technology may be further improved, and that the measurement accuracy may be better than 5 % of full scale. Rod extrusion experiments allowed the quality of the finite element modelling approach to be evaluated. The code ALMA2π was used, and material data of the Zener-Hollomon flow rule have been obtained by compression testing. Simulated and measured results of the ram force, die face pressure and liner force generally differed less than 10 %. The estimated die outlet temperature change was systematically 10 ºC too high. As there are significant differences between extrusion and compression testing, the use of material data from compression testing amounts to an extrapolation of data. Experiments did not demonstrate that the approach is unacceptable, but plots showing the deviation between experimental and estimated ram force and outlet temperature data indicated that there are a number of parameter combinations that are equally good as or even better than those obtained through compression testing. Very high accuracy determination of flow parameters may be difficult. Measurement errors significantly complicate matters. The pressure sensors may be used to study practical extrusion problems and to establish a better understanding of metal flow and the significance of die deformation. Thin strip extrusion experiments were performed to gain insight into the thermo-mechanics of flow instability (buckling). The feasibility of performing pressure measurement during the extrusion of thin strips was demonstrated, but sensors were not properly calibrated. The first round of the experiments was run with a die outlet 78.5 mm wide and 1.7 mm thick, and a container diameter of 100 mm. During extrusion of AA6060 flow instability phenomena were not encountered. A second round of experiments was performed with non-instrumented dies and somewhat thinner profiles (1.1 and 1.4 mm). Flow instability in the form of buckling was provoked for high outlet temperatures, and many replicate experiments were performed. The shape of the buckled thin strip was also measured continuously with a laser triangulation technique during extrusion at high speed. Due to limitations related to the experimental set-up, neither the resolution nor the accuracy of the approach was entirely satisfactory. Nonetheless, the feasibility of the approach was demonstrated, and it is quite possible to improve the measurement technique. Capacitive pressure measurement techniques have been combined with methods for measuring the deformation of the mandrel and the straining of the bridges to study the behaviour of dies during tube extrusion. Capacitec capacitive probes were used to measure bridge strains. High temperature Kyowa strain gauges were also used for the same purpose. The die mandrel deflection was measured by conventional displacement transducers connected to the mandrel and the die cap by rods. Only measurements by the capacitive sensors proved sufficiently reliable during measurements. The study revealed that the state of stress in dies for hollow profiles may be very close to critical. The die face pressure at the top face of the mandrel exceeded 500 MPa. Two rounds of industrial experiments were performed with a U-profile that proved most difficult to extrude. In the first round of experiments, the flow stability was not satisfactory, and plugging of the outlet ruined experiments. The second round was more successful, and sensors were used to record the die face pressure on-line. The experiments demonstrated the feasibility of industrial experiments, but clearly indicated that further development of the sensors should be performed. It is important that sensors are made more durable, and that calibration techniques are further developed. Practical die designs that allow simple integration of sensors in the press should be developed.